As having high operation voltage and high energy density, non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, have come into practical use as power sources for driving portable electronic instruments such as cell phones, laptop computers and video camcoders. Non-aqueous electrolyte secondary batteries have been in the mainstream of small-sized secondary batteries and the production volume thereof is on the increase.
As a positive electrode active material of non-aqueous electrolyte secondary batteries, a lithium-containing composite oxide with a 4V-class high voltage has been in use. Typical lithium-containing composite oxides include LiCoO2 and LiNiO2 which have a hexagonal structure, and LiMn2O4 which has a spinel structure. Among them, the mainstream one has been LiCoO2 since it has high operation voltage and high energy density.
For negative electrodes used have been carbon materials capable of absorbing and desorbing lithium ions. Among those materials, the prevailing one is a graphite material with a flat discharge potential and high specific density (capacity density).
Recently, not only the development of non-aqueous electrolyte secondary batteries for small-sized customer application, but the development of large-sized non-aqueous electrolyte secondary batteries, having a large capacity for power storage, electric vehicles and the like, has been accelerated. For example, hybrid electric vehicles (HEVs) loaded with nickel-metal hydride batteries have already been in mass production and available in the market. Further, HEVs loaded with non-aqueous electrolyte secondary batteries in place of nickel-metal hydride batteries are under development.
Batteries used for HEVs and the like are required to have high input/output power performance for immediate power-assistance of an engine and regeneration of energy, and in this respect, they are significantly different from batteries for small-sized customer application. Preference is therefore given to higher output power over higher energy density. For achieving higher output power, it is necessary to minimize internal resistance of a battery. For this reason, attempts have been made not only to develop and select active materials and an electrolyte, but also to improve current collecting structure of an electrode, reduce component resistance, and make an electrode thinner and longer so as to increase a reactive area of the electrode.
In non-aqueous electrolyte secondary batteries for small-sized customer application, normally, a positive electrode active material comprising LiCoO2 is combined with a negative electrode active material comprising a graphite material. In large-sized non-aqueous electrolyte secondary batteries, however, a combination of a positive electrode active material comprising another lithium-containing composite oxide besides LiCoO2 and a negative electrode active material comprising a low crystalline carbon material, such as a non-graphitizable carbon material, has been considered as promising.
Examples of negative electrode materials that have hitherto been proposed may include: a graphite material primarily used in small-sized customer application (Japanese Laid-Open Patent Publication No. 2000-260479); a non-graphitizable carbon material with low crystallinity (Japanese Laid-Open Patent Publication No. 2000-200624); and a pseudo-graphite material with a controlled graphitization degree (Japanese Laid-Open Patent Publication No. 2000-260480).
However, a graphite material has a structure of hexagonal layers regularly arranged in a c-axis direction. During charge, lithium is intercalated between the graphite layers to extend each interval of the layers, leading to expansion of the graphite. Stress to be applied to the graphite associated with the expansion becomes considerably large when charge with a large-current pulse is repeated. This causes a gradual decrease in charging capability (acceptance of charge) of the graphite, thereby increasing deterioration in battery cycles.
In charge/discharge reactions of non-graphitizable carbon, on the other hand, there occurs almost no intercalation of lithium between the graphite layers during the charge. This is because most of lithium is inserted into pores of the non-graphitizable carbon. For this reason, just a small amount of stress is applied to the non-graphitizable carbon due to the expansion and shrinkage thereof through charging/discharging. Since non-graphitizable carbon has lower conductivity than graphite carbons, however, the internal resistance thereof increases during discharge when lithium is deintercalated. This tendency becomes conspicuous especially when large-current discharge is repeated.
A pseudo-graphite material is a carbon material with a relatively high graphitization degree since the crystallite thereof in a c-axis direction has a thickness “Lc” of not smaller than 60 nm and smaller than 100 nm. Therefore, the charging capability of the pseudo-graphite material tends to decrease in almost the same manner as in the case of using graphite.
Moreover, another carbon material has been proposed wherein, in a wide-range X-ray diffraction pattern, a ratio of an intensity I (101) of a peak attributed to a (101) crystal face to an intensity I (100) of a peak attributed to a (100) crystal face satisfies: 0.7≦I (101)/I (100)≦2.2 (Japanese Laid-Open Patent Publication No. Hei 6-275321). Although this carbon material has a developed hexagonal layered structure, there exists slight misalignment or torsion between layers, as compared with natural graphite having a graphite structure proximate to a monocrystal. It is described that the ratio of I (101) to I (100) is preferably 0.8 or larger and that a favorable characteristic is exhibited when the ratio of I (101) to I (100) is 1.0 or larger. This proposal however does not relate to non-aqueous electrolyte secondary batteries with high output power. Therefore, the negative electrode and the positive electrode thereof are as thick as 180 μm and 270 μm, respectively, and the electrode area per battery capacity of 1 Ah is not smaller than 125 cm2 and not larger than 500 cm2.
Furthermore, a combination of two kinds or more of carbon materials, a spacing (d002) of which in a c-axis direction is not larger than 0.34 nm, has also been proposed (Japanese Laid-Open Patent Publication No. Hei 9-171814). It is proposed that the preferable (d002) of a carbon material having the largest mean particle size is less than 0.337 nm whereas the preferable (d002) of a carbon material having the smallest mean particle size is not smaller than 0.337 nm and not larger than 0.34 nm. However, this proposal is aimed at achieving a non-aqueous electrolyte secondary battery with high energy density and long cycle life, and does not relate to a high input/output power battery.
Next, charge/discharge cycle conditions of high output power non-aqueous electrolyte secondary batteries significantly differ from those of normal batteries for small-sized customer application. In general, a high output power non-aqueous electrolyte secondary battery is not sequently charged or discharged between a fully discharged state and a fully charged state. A typical charging/discharging operation of this battery is to repeat pulse charge/discharge on the second time scale with a 50-60% charged state taken as a base point. Such a battery is required to be capable of repeating pulse charge and pulse discharge with various currents from a small current to a large current.
When pulse charge/discharge are repeated for a long period of time, however, the charging/discharging capability of the electrode active material deteriorates and the capacity decreases as well as the internal resistance of the battery increases. As a result, in the application of HEVs, for example, power-assisting and regenerating capability become insufficient. Accordingly, in the technical development of high output power non-aqueous electrolyte secondary batteries, lengthening lifetime is as important as improving output power.